Studies are proposed to examine further the transmitter-specific cell classes, afferent brainstem pathways, and synaptic circuitry of the cat superior colliculus and to study how these are altered by monocular deprivation. Three aims are proposed for the current grant period. The first aim is to study further the synaptic connections and molecular properties of putative GABAergic neurons in the superior colliculus. We propose to: (1) determine whether one or more types of GABA labeled cell project to nuclei of the dorsal thalamus; (2) determine whether cells labeled by the anti-GABA are also labeled by peptide antibodies, including cholecystokinin, somatostatin, and substance P; (3) determine whether specific GABA cell classes are also labeled by antibodies to the calcium binding proteins parvalbumin and calbindin D; (4) examine the distribution of an antibody to the GABAa receptor complex and determine whether the receptor is associated with specific classes of GABA cell. Antibodies will be localized using double-label immunocytochemistry at the light and electron microscope levels and measured using computer-assisted quantitative image analysis. The second aim is to study the synaptic organization of norepinephrine (NE) inputs to the superior colliculus. We propose to: (1) determine the synaptic connections between fibers labeled by an antibody to dopamine- Beta-hydroxylase and specific sensory afferents and intrinsic neurons of SC using serial section reconstruction. (3) determine whether the density, distribution, or synaptic organization of NE fibers is altered by monocular deprivation (MD) as measured with an image analyzer. The third aim is to examine the synaptic organization of acetylcholine (ACh) pathways to the superior colliculus. We propose to: (1) examine the synaptic relationships of fibers labeled by an antibody to choline acetyltransferase (ChAT) using serial section reconstruction; (2) determine the distribution of an antibody to the AChR nicotinic receptor with the light and electron microscopes; (3) determine whether there are alterations in the distribution or density of ChAT labeled fibers or the AcHR nicotinic receptor produced by MD. These results will characterize in detail the transmitter-specific synaptic circuitry which underlies cellular physiology in the superior colliculus and determine how specific synapses and receptors are altered by developmental abnormalities.